In many cases, an electric power system comprises electric energy sources each of which can be individually activated and deactivated so that different ones of the electric energy sources can be active at different moments of time. The electric power system can be for example a part of an electric system of a ship or a ferry in which case the electric power system is typically arranged to supply electric power to one or more propulsion motors, to an alternating voltage network of the ship or ferry, and/or to other loads such as e.g. one or more bow thruster motors. The motors are advantageously alternating current “AC” motors which are supplied with inverters for converting direct voltage of a direct voltage rail into alternating voltages suitable for the AC-motors. Each of the electric energy sources can be for example a combustion engine driven generator, a battery, or a fuel cell.
An inherent challenge related to electric power systems of the kind described above is the need to control the electric energy sources so that an optimal or at least a near optimal subset of the electric energy sources is active in each operating situation of the electric power system. Factors which have significance when selecting electric energy sources to be activated or deactivated are: electric power that needs to be produced by the electric power system, operating costs of the electric energy sources, efficiencies of the electric energy sources, starting characteristics of the electric energy sources, serviceable life times of the electric energy sources, and/or other factors such as an operation mode of the electric power system. In a case of a ship or ferry, the operation modes can be e.g. open sea operation and harbor operation. In this document, the term efficiency means a ratio between output power of an electric energy source and a sum of the output power and losses of the electric energy source. The starting characteristics may mean for example time and energy needed for activating an energy source to produce electric power. For example, a charged battery does not need time and externally given energy for being activated to produce electric energy but e.g. a combustion engine driven generator needs time and externally given energy for starting the combustion engine.